The Improved Cylinder choke is the least constrictive of the ones you have, and the Full is the most constrictive. Since your shotgun is threaded for chokes it's most likely smooth bore. Get rifled slugs. I'd probably use the Improved Cylinder choke for shooting slugs, but it more than likely won't damage anything to use the others. Changing chokes will, however, probably change the point of impact, so decide which one you want to continue to use for slugs and stick with it.
Hope that helps.
IDPA/USPSA/3 Gun Junkie
Jack of all trades, master of none.